Unscheduled shutdowns in a power network normally cause a large loss of income to the network operator. When a shutdown is caused by damaged or malfunctioning network components, there will also be additional costs for the replacement and/or repair of said network components. Different types of surge arresters are today used in switchgears in order to protect equipment against incoming overvoltages. A surge arrester is connected between a live wire and ground and may be designed as a gapless surge arrester with series connected zinc oxide varistors. When the voltage level gets too high in the live wire, the surge arrester will allow the electric current to be conducted to ground, whereby the overvoltage is reduced.
The number of overvoltages a surge arrester is subjected to may be recorded by means of a so-called surge counter, which is connected to the grounding cable of the surge arrester. The surge counter gives information about the extent to which the installation is subjected to overvoltages and serves as a monitoring device for the surge arrester. An exceptional number of recorded overvoltages during a certain period of time indicates that the surge arrester should be inspected and possibly subjected to maintenance. A surge counter is normally provided with a relay that counts the number of surges passing through the surge arrester. The surge counter is normally sensitive to the charge content or amplitude of the impulse current through the surge arrester caused by the overvoltage and the counting function is activated when certain predetermined values with respect to the amplitude and/or duration of an electric current pulse are exceeded. A surge counter often has a display which shows the number of recorded surges.
If a porcelain housed surge arrester is subjected to severe pollution, the surge arrester might be partially heated due to transient variations in the voltage distribution caused by external soiling of the casing of the surge arrester. In such a case, the casing of the surge arrester should be cleaned. The layer of soil accumulated on the casing contains conductive and semiconductive particles, which cause a leakage current to flow through the casing. The leakage current flowing through the casing will be included in the total leakage current through the surge arrester. A sudden increase of the total leakage current through the surge arrester indicates that the casing has been soiled and needs to be cleaned. Monitoring devices comprising means for the combined recording of total leakage current through a surge arrester and surge counting have therefore come into use.
In a zinc oxide surge arrester, the varistors are continuously subjected to an operating voltage which causes a continuous electric current in the order of 1 mA to flow through the surge arrester. This electric current is under normal operating conditions essentially capacitive but does also contain a smaller resistive component. It is only changes in the resistive current component that can indicate possible changes in the characteristics of the surge arrester. Zinc oxide surge arresters have a very long service life, but its varistor blocks may undergo a slow deterioration resulting in a gradual increase of the resistive leakage current. When the resistive leakage current has increased to a certain level, there is a risk that a thermal racing process occurs, which results in the destruction of the varistor blocks. It is therefore of interest to check the resistive leakage current through the surge arrester. This may be done by means of a so-called field probe. The field probe is intended to pick up the electric field from the power network to which the surge arrester is connected. Based on measuring values related to the electric current flowing between the field probe and ground and measuring values related to the electric current flowing between the surge arrester and ground, the resistive leakage current of the surge arrester can be calculated with a specific method of calculation, the so-called Method B2 (“Third order harmonic analysis with compensation for harmonics in voltage” (Amendment 1, Section 6 IEC 60099-5)), as described in U.S. Pat. No. 7,005,863 B2.